INTRODUCTION. For cementless TKA, highly crosslinked UHWMPE is traditionally used with modular components because of manufacturing and sterilization complexities of monoblock metal-backed components. However, it would be very useful to have a highly crosslinked UHMWPE monoblock metal-backed cementless component to address historical clinical issues. The purpose of this study was to evaluate the wear properties of a unique process for achieving a monoblock metal-backed cementless component featuring highly crosslinked polyethylene to standard highly crosslinked UHWMPE. MATERIALS AND METHODS. The knee system used for testing consisted of cobalt chrome femoral components and tibial trays (Triathlon®, Stryker Orthopaedics, Mahwah, NJ). Modular tibial inserts were machined from GUR 1020 polyethylene that was irradiated to 30 kGy and annealed three times (Modular, n=5) (X3, Stryker Orthopaedics, Mahwah, NJ). Monoblock tibias were direct compression molded to a metal substrate and then irradiated to 30 kGy and annealed three times. For the purposes of this test, the polyethylene was removed from the monoblock component and machined into a standard tibial insert (Monoblock, n=5). A 6-station knee simulator was utilized for testing (MTS, Eden Prairie, MN). All motion and loading was computer controlled and waveforms followed ISO 14243-3 [1]. Testing was conducted at a frequency of 1 Hz for 3 million cycles. The lubricant used was Alpha Calf Fraction serum (Hyclone Labs, Logan, UT) diluted to 50% with a pH-balanced 20-mMole solution of deionized water and EDTA [2]. The serum solution was replaced and inserts were weighed for gravimetric wear at least every 0.5 million cycles. Standard test protocols were used for cleaning, weighing and assessing the wear loss of the tibial inserts [3]. Soak control specimens were used to correct for fluid absorption with weight loss data converted to volumetric data (by material density). Statistical analysis was performed using the Student's t-test with significance determined at the 95% confidence level (p < 0.05). RESULTS. The average volume loss and volumetric wear rates at 3 million cycles are represented in Figure 1 and Figure 2. Group 1: Highly Crosslinked UHMWPE had a wear rate of 2.8±0.6 mm. 3. /mc, where the Group 2: Monoblock Highly Crosslinked UHMWPE was 2.8±0.2 mm. 3. /mc. There was no statistical difference (p = 0.95). CONCLUSION. The results of testing show that the Monoblock Highly Crosslinked UHMWPE material is the same in wear performance relative to Modular Highly Crosslinked UHMWPE material. These results suggest that this new material may have applications in monoblock implant design applications but further testing for specific applications is needed

Abstract

Cemented total hip arthroplasty (THA) in the younger patient has historically been associated with higher wear and revision rates. We carried out a retrospective study of a prospectively collected database of patients at Wrightington hospital undergoing cemented THA under 55 years of age to determine acetabular wear and revision rates.

Between August 2005 and December 2021 a cohort of 110 patients, 56 males and 54 females, underwent Cemented Total Hip Replacement through a posterior approach. Mean age at operation was 50yrs (35–55). The mean follow up was 6 years 9 months (0–16 years). 3 patients were lost to follow-up. Of the remaining 107 patients, Conventional and cross lined polyethylene were used in 54 and 53 patients respectively. Ceramic heads were used in 102 patients. 22.225mm and 28mm heads were used in 60 and 47 patients respectively. Clinical outcomes were assessed by Merle d'Aubigne and Postel scores which showed significant functional improvement. Linear wear was measured on plain radiographs using TRAUMA CAD and cup loosening was assessed by classification of Hodgkinson et al.

No cases were revised during the observed follow up period. The mean wear rate in conventional and crosslinked polyethylene cups were 2.31mm (0.1–4.6) and 1.02mm (0.1–2.6) respectively.

Cemented THA with both conventional and crosslinked polyethylene provides excellent survival rates in adults under the age of 55 years and crosslinked polyethylene may further improve these results due to improved wear rates.

Since its introduction in total hip replacements in the 1960's, Ultra High Molecular Weight Polyethylene (UHMWPE) has played a major role as a bearing component material for joint arthroplasty. Concerns were raised when issues of wear resistance became apparent, and therefore Highly Crosslinked Polyethylenes were introduced. Such materials undergo a thermal treatment to quench the free radicals and reduce progressive oxidation. However, said thermal treatment weakens the material mechanical properties and hence the use of antioxidants has been proposed and implemented in clinical use, mainly Vitamin-E. This can be added to the material before or after irradiation. If it is done before, part of the anti-oxydant is consumed during irradiation and so will not be available for its main purpose, and part reacts before irradiation with the free radicals thus reducing the crosslinking effect. If it is added after irradiation, high temperatures are required in order to diffuse it in the bulk material, and anyway the surface will be mainly rich in antioxidant. However, Vitamin-E tends to neutralize the free radicals on the oxidized lipid chain present in our body fluids and so in direct contact with the prosthetic components: such mechanism reduces the Vitamin-E quantity available for anti-oxidation purposes in the long run. A UHMWPE doped with Hindered Amine Light Stabilizer (HALS) has been developed and tested for applications in large joint replacements where highest resistance to wear and tough mechanical properties are simultaneously required, such as tibial inserts for knee joints or acetabular inserts for large diameter heads. Mechanical and biocompatibility tests were run in accordance with ASTM F 2565-06 and ISO 10993-1 with successful results and good reproducibility. In particular, electro spin resonance exhibited a very high level of free radicals in the three samples, which confirms the properties of this new material. Free radicals are the result of the activation of the HALS molecules during irradiation, creating nitroxide radicals that will destroy the residual alkyl radicals responsible for the oxidation before and after implantation. Biocompatibility tests proved absence of cytotoxicity, sensitization, irritation, genotoxicity or pyrogenic reactions. The possible future applications for this new material in the arthroplasty field will be discussed along with the expected advances and advantages

The benefits of HXLPE in total knee arthroplasty (TKA) have not been as evident as total hip arthroplasty (THA). A systematic review and meta-analysis to assess the impact of highly-crosslinked polyethylene (HXLPE) on TKA outcomes compared to conventional polyethylene (CPE) is described.

All studies comparing HXLPE with CPE for primary TKA were included for analysis. The minimum dataset included revision rates, indication for revision, aseptic component loosening and follow-up time. The primary outcome variables were all-cause revision, aseptic revision, revision for loosening, radiographic component loosening, osteolysis and incidence of radiolucent lines. Secondary outcome measures included postoperative functional knee scores. A random-effects meta-analysis allowing for all missing data was performed for all primary outcome variables.

Six studies met the inclusion criteria. In total, there were 2,234 knees (1,105 HXLPE and 1,129 CPE). The combined mean follow-up for all studies was 6 years. The aseptic revision rate in the HXLPE group was 1.02% compared to 1.97% in the CPE group. There was no difference in the rate of all-cause revision (p = 0.131), aseptic revision (p = 0.298) or revision for component loosening (p = 0.206) between the two groups. Radiographic loosening (p = 0.200), radiolucent lines (p = 0.123) and osteolysis (p = 0.604) was similar between both groups. Functional outcomes were similar between groups.

The use of HXLPE in TKA yields similar results for clinical and radiographic outcomes when compared to CPE at midterm follow-up. HXLPE does not confer the same advantages to TKA as seen in THA.

With the introduction of highly crosslinked polyethylene (HXLPE) in total hip arthroplasty (THA), orthopaedic surgeons have moved towards using larger femoral heads at the cost of thinner liners to decrease the risk of instability. Several short and mid-term studies have shown minimal liner wear with the use HXLPE liners, but the safety of using thinner HXPLE liners to maximize femoral head size remains uncertain and concerns that this may lead to premature failure exist. Our objective was to analyze the outcomes for primary THA done with HXLPE liners in patients who have a 36-mm head or larger and a cup of 52-mm or smaller, with a minimum of 10-year follow-up. Additionally, linear and volumetric wear rates of the HXLPE were evaluated in those with a minimum of seven-year follow-up. We hypothesized that there would be minimal wear and good clinical outcome.

Between 2000 and 2010, we retrospectively identified 55 patients that underwent a primary THA performed in a high-volume single tertiary referral center using HXLPE liners with 36-mm or larger heads in cups with an outer diameter of or 52-mm or smaller. Patient characteristics, implant details including liner thickness, death, complications, and all cause revisions were recorded. Patients that had a minimum radiographic follow-up of seven years were assessed radiographically for linear and volumetric wear. Wear was calculated using ROMAN, a validated open-source software by two independent researchers on anteroposterior X-rays of the pelvis.

A total of 55 patients were identified and included, with a mean age of 74.8 (range 38.67 - 95.9) years and a mean BMI of 28.98 (range 18.87 - 63-68). Fifty-one (94.4%) of patients were female. Twenty-six (47.7%) patients died during the follow-up period. Three patients were revised, none for liner wear, fracture or dissociation. Twenty-two patients had a radiographic follow-up of minimum seven years (mean 9.9 years, min-max 7.5 –13.7) and were included in the long-term radiographic analysis. Liner thickness was 5.5 mm at 45 degrees in all cases but one, who had a liner thickness of 4.7mm, and all patients had a cobalt-chrome head. Cup sizes were 52mm (n=15, 68%) and 50mm (n=7, 32%). Mean linear liner wear was 0.0470 mm/year (range 0 - 0.2628 mm) and mean volumetric wear was 127.69 mm3/year (range 0 - 721.23 mm3/year).

Using HXLPE liners with 36-mm heads or bigger in 52-mm cups or smaller is safe, with low rates of linear and volumetric wear in the mid to long-term follow-up. Patients did not require revision surgery for liner complications, including liner fracture, dissociation, or wear. Our results suggest that the advantages of using larger heads should outweigh the potential risks of using thin HXLPE liners.

Increased femoral head size reduces the rate of dislocation after total hip arthroplasty (THA). With the introduction of highly crosslinked polyethylene (HXLPE) liners in THA there has been a trend towards using larger size femoral heads in relatively smaller cup sizes, theoretically increasing the risk of liner fracture, wear, or aseptic loosening. Short to medium follow-up studies have not demonstrated a negative effect of using thinner HXLPE liners. However, there is concern that these thinner liners may prematurely fail in the long-term, especially in those with thinner liners. The aim of this study was to evaluate the long-term survival and revision rates of HXLPE liners in primary THA, as well as the effect of liner thickness on these outcomes. We hypothesized that there would be no significant differences between the different liner thicknesses.

We performed a retrospective database analysis from a single center of all primary total hip replacements using HXLPE liners from 2010 and earlier, including all femoral head sizes. All procedures were performed by fellowship trained arthroplasty surgeons. Patient characteristics, implant details including liner thickness, death, and revisions (all causes) were recorded. Patients were grouped for analysis for each millimeter of PE thickness (e.g. 4.0-4.9mm, 5.0-5.9mm). Kaplan-Meier survival estimates were estimated with all-cause and aseptic revisions as the endpoints.

A total of 2354 patients (2584 hips) were included (mean age 64.3 years, min-max 19-96). Mean BMI was 29.0 and 47.6% was female. Mean follow-up was 13.2 years (range 11.0-18.8). Liner thickness varied from 4.9 to 12.7 mm. Seven patients had a liner thickness <5.0mm and 859 had a liner thickness of <6.0mm. Head sizes were 28mm (n=85, 3.3%), 32mm (n=1214, 47.0%), 36mm (n=1176, 45.5%), and 40mm (n=109, 4.2%), and 98.4% were metal heads. There were 101 revisions, and in 78 of these cases the liner was revised. Reason for revision was instability/dislocation (n=34), pseudotumor/aseptic lymphocyte-dominant vasculitis associated lesion (n=18), fracture (n=17), early loosening (n=11), infection (n=7), aseptic loosening (n=4), and other (n=10). When grouped by liner thickness, there were no significant differences between the groups when looking at all-cause revision (p=0.112) or aseptic revision (p=0.116).

In our cohort, there were no significant differences in all-cause or aseptic revisions between any of the liner thickness groups at long-term follow-up. Our results indicate that using thinner HXPE liners to maximize femoral head size in THA does not lead to increased complications or liner failures at medium to long term follow-up. As such, orthopedic surgeons can consider the use of larger heads at the cost of liner thickness a safe practice to reduce the risk of dislocation after THA when using HXLPE liners.

Contemporary polyethylene liners for total hip replacements were introduced in the late 1990's to address osteolysis associated with wear of conventional polyethylene. Every major device manufacturer introduced an “enhanced polyethylene”. In the ensuing decade plus, every major arthroplasty meeting had presentations and debates about the wear resistance and mechanical properties of these new polymers. The results have been remarkable and now with 17 to 18 years of use in patients, we have yet to see clinically significant osteolysis in our patients regardless age or activity level.

The results can be summarised as follows: All currently commercially available highly crosslinked polyethylenes produced by major device companies have demonstrated a reduction in wear and osteolysis. At the 2016 Closed Meeting of The Hip Society, none of the surgeons attending had seen a clinically significant case of osteolysis associated with highly crosslinked polyethylene. Registry data demonstrates the superiority of the highly crosslinked materials over conventional polyethylenes.

Historical concerns over a reduction in mechanical properties have not been borne out in clinical studies. Although highly crosslinked polyethylene liner fractures have been reported, they are rare and probably related to specific designs or surgical technique issues. It is important to remember that there were rare cases of fracture of conventional polyethylene as well.

With currently reported wear rates of the enhanced polyethylenes, polyethylene thickness is unlikely to be a factor in long-term durability with well-designed sockets. Bench data has demonstrated that polyethylene thickness is not a risk factor for wear or fracture if well supported by the metal shell. Thin unsupported polyethylene is at risk for fracture.

Although the new anti-oxidant polyethylenes (eg. Vitamin E) have performed well in wear studies, there is no clinically available evidence to support their use based on enhanced fracture toughness.

Do we need new polyethylene? Is there a clinical problem with first generation crosslinked polyethylene (XLPE)? Are we being duped into believing that doped polyethylene will solve a problem?

Clinical failures of polyethylene bearing total hip replacements are related to wear and the mechanical properties of the polyethylene. Wear is primarily related to crosslinking. Wear failures are secondary to periprosthetic osteolysis while mechanical failure causes cracking of thin polyethylene. Use of large femoral heads that reduce dislocation may increase wear and mechanical failure in the second decade of XLPE use.

There is no question that XLPE has reduced 2-dimensional (2D) head penetration, volumetric penetration and periprosthetic osteolysis with traditional 28 mm head sizes. Reported 2D penetration rates are 0.03–0.07 mm/year and clinically important polyethylene wear induced osteolysis is nonexistent. However, larger heads with the same 2D head penetration will generate more volumetric debris and could cause osteolysis.

There is no question that retrieved XLPE components have low levels of oxidation at the time of explant. While this is unexpected, the levels are well below levels reported with traditional polyethylene. It remains to be seen if these levels of oxidation will cause mechanical failures.

Currently available versions of polyethylene have focused on eliminating oxidation induced mechanical property reduction and not additional wear reduction. This is accomplished with Vitamin E doping or blending. While the local effects of Vitamin E polyethylene particles at the cellular level have been studied the clinical effect of these chemically new particles remains to be seen.

This author believes that long term volumetric wear with large head size is a greater concern than reduced mechanical properties secondary to in-vivo oxidation. New polyethylene development needs to focus on additional wear reduction. Can we afford to pay more for a new polyethylene in a value based healthcare environment?

Wear of the tibial polyethylene liner of total knee arthroplasty (TKA) is complex and multifactorial. The issues involved include those of implant design and locking mechanism, surgical-technical variability, and patient weight and activity level. However, tibial polyethylene fabrication, including bar stock, amount of irradiation, quenching of free radicals, and sterilization may also be factors in the long-term survival of TKA. Highly crosslinked polyethylene is now widely used in total hip arthroplasty, but its value and use in TKA is a subject of great controversy. In making a decision to use these products, the surgeon should consider multiple sources of evidence: in-vitro wear testing; clinical cohort studies; randomised controlled trials; registry survival data; and retrieval analyses. The two questions to be asked are: is there a value or benefit in the use of these new polyethylenes, and what are the risks involved with the use of these products?

Laboratory testing, generally to 5 million cycles, has shown a significant decrease in tibial polyethylene wear of several products, with both cruciate-retaining and substituting designs, and under adverse conditions. Retrospective cohort studies and RCTs comparing conventional and highly crosslinked polyethylene have shown little difference between the two products at mean follow-up times of 5 years. One registry study showed no difference in the rate of revision at short-term follow-up, but the Australian Joint Replacement Registry in 2014 did report a decreased rate of revision for loosening and osteolysis in “young patients” with one particular tibial polyethylene.

The risks of the use of highly crosslinked polyethylene include fracture (the tibial liner, PS post, and patella pegs), and particle size-reactivity. However, these risks appear to be quite low. Retrieval data shows lower damage scores with certain polyethylenes, and variable changes in the oxidation score.

At present, the data does not support the widespread or routine use of highly crosslinked or antioxidant polyethylene in TKA. However, consideration should be given for use of certain products in young and active patients. Longer-term follow-up will ultimately determine the role of highly crosslinked polyethylene in TKA.

INTRODUCTION

Since the early 2000s, highly cross-linked (HXL) UHMWPE's have become a popular option with multiple experimental and clinical studies showing that gamma or electron radiation doses between 50–100kGY reduce wear and potentially extend the bearing life of UHMWPE. However, the increased wear resistance came at a compromise to mechanical properties due to the cross-linking process. Vitamin E has been added to some HXL UHMWPE materials to offer a solution to the compromise by increasing oxidation resistance and maintaining sufficient fatigue strength. However, limited data is available on the effect of the fabrication process, especially the method of irradiation, on the properties of the Vitamin E blended HXL UHMWPE. The purpose of this study was to evaluate the effects of adding the antioxidant vitamin E to highly crosslinked UHMWPE on wear rates.

Introduction

Highly crosslinked, ultra-high molecular weight polyethylene (HXLPE) acetabular liners inherently have a risk of fatigue failure associated with femoral neck impingement. One of the potential reasons for liner failure was reported as crosslinking formulations of polyethylene, increasing the brittleness and structural rigidity. In addition, the acetabular component designs greatly affect the mechanical loading scenario, such as the offset (lateralized) liners with protruded rim above the metal shells, which commonly induce a weak resistance to rim impingement. The purpose of the present study was to compare the influence of the liner offset length on the impingement resistance in the annealed (first generation) and vitamin E-blended (second-generation) HXLPE liners with a commercial design.

Introduction

Total hip replacement with metal-on-polymer (MoP) hip prostheses is a successful treatment for late-stage osteoarthritis. However, the wear debris generated from the polymer acetabular liners remains a problem as it can be associated with osteolysis and aseptic loosening of the implant. This has led to the investigation of more wear resistant polymers in orthopaedics. Cross-linked polyethylene (XLPE) is now the gold-standard acetabular liner material. However, we asked if carbon fibre reinforced polyether ether ketone (CFR-PEEK) might be a lower wear material. In addition, we sought to understand the influence of contact stress on the wear of both XLPE and CFR-PEEK as this has not previously been reported.

Highly crosslinked polyethylene (XLPE) was introduced to decrease peri-prosthetic osteolysis related to polyethylene wear, a major reason for revision of total hip arthroplasty. There are few reports of wear and osteolysis at 10 years post-operatively. We asked the following questions: (1) What are the linear and volumetric wear rates of one remelted XLPE at 10–14 years using the Martell method? (2) What is the relationship between volumetric wear, femoral head size, and osteolysis? (3) What is the incidence of osteolysis using conventional radiographs with Judet views and the Martell method?

Methods We evaluated a previously reported cohort of 84 hips (72 patients) with one design of an uncemented acetabular component and one electron-beam irradiated, remelted XLPE at a mean follow-up of 11 years (range 10 to 14 years). Measurements of linear and volumetric wear were performed in one experienced laboratory by the Martell method and standard radiographs, with additional Judet views, were used to detect peri-prosthetic osteolysis. Statistical analysis of wear and osteolysis compared to head size was performed.

Results The mean linear wear rate by the first-to-last method was 0.024 mm/year (median, 0.010 mm/year) and the mean volumetric wear rate by this method was 12.2 mm³/year (median, 3.6 mm³/year). We found no association between femoral head size and linear wear rate. However, there was a significant relationship between femoral head size and volumetric wear rates, with 36/40 mm femoral heads having significantly higher volumetric wear (p=0.02). Small osteolytic lesions were noted in 12 hips (14%), but there was no association with head size, acetabular component position, or linear or volumetric wear rates.

Conclusion This uncemented acetabular component and this particular remelted XLPE had low rates of linear and volumetric wear. Small osteolytic lesions were noted at 10 to 14 years, but were not related to femoral head size, linear or volumetric wear rates.

The purpose of this investigation is to assess the rate of wear the effect once the “bedding in period”/ poly creep had been eliminated. Creep is the visco-elastic deformation that polyethylene exhibits in the first 6–12 weeks. We also assessed the wear pattern of four different bearing couples in total hip arthroplasty (THA): cobalt-chrome (CoCr) versus oxidized zirconium (OxZir) femoral heads with ultra-high molecular weight polyethylene (UHMWPE) versus highly-crosslinked polyethylene (XLPE) acetabular liners.

This was a randomized control study involving 92 patients undergoing THA. They were randomized to one of four bearing couples: (1) CoCr/UHMWPE (n= 23), (2) OxZir/UHMWPE (n=21), (3) CoCr/XLPE (n=24), (4) OxZir/XLPE (n=24). Patients underwent a posterior approach from one of three surgeons involved in the study. All patients received a porous-coated cementless acetabular shell and a cylindrical proximally coated stem with 28 mm femoral heads. Each patient was reviewed clinically and radiographically at six weeks, three and 12 months, two, five and 10 years after surgery. Standardized anteroposterior and lateral radiographs were taken. All polyethylene wear was measured by an independent blinded reviewer. Linear and volumetric wear rates were measured on radiographs using a validated computer software (Polyware Rev. 5). Creep was defined as the wear at 6 or 12 weeks, depending on if there was a more than 10% difference between both measurements. If a greater than 10% difference occurred than the later period's wear would be defined as creep.

72 hips were included in analysis after exclusion of seven revisions, three deaths and 10 losses to follow-up. The annual linear wear rates (in mm/y) at 10 years were (1) 0.249, (2) 0.250, (3) 0.074 and (4) 0.050. After adjusting for creep these rates become were (1) 0.181, (2) 0.142, (3) 0.040 and (4) 0.023. There is statistical differences between raw and adjusted linear wear rates for all bearing couples. The percentage of the radiographically measured wear at 10 years due to creep is (1) 30% (2) 44%, (3) 58.5% and (4) 51.5% with significant differences in couples with XLPE versus those with UHMWPE. There was no significant correlation between age, gender, cup size, tilt, planar anteversion and the linear or volumetric wear rates.

The linear wear rate of both UHMWPE and XLPE are even lower thxdsxzan previously described when creep is factored out. XLPE has again demonstrated far superior linear wear rates at 10 years than UHMWPE. There were no significant differences in wear rate at 10 years between CoCr and OxZir, this may be due to an underpowered study. XLPE exhibits proportionally more creep than UHMWPE within the first 6–12 weeks and accounts for more of the total wear at 10 years as measured radiographically at the end period.

Introduction

Cross-linked polyethylene in total hip arthroplasty has demonstrated excellent long-term wear resistance, leading to its acceptance as the standard bearing used in hip replacement. Adoption in knee replacement has been tentative, as the cross-linking process can decrease the polyethylene mechanical properties. The current study's purpose was examining survivorship of a fixed bearing knee replacement system featuring a moderately cross-linked polyethylene (MXLK) bearing, a cobalt chrome (CoCr) tibial tray with a highly polished top surface, and a new polyethylene-to-tray locking mechanism. The MXLK is made of ultra-high molecular weight GUR1020 resin irradiated with 5 Mrad gamma radiation, followed by a free radical quenching remelting annealing process, above the 135 degree melting point, that provides wear and fatigue resistance, and oxidative stability.

INTRODUCTION

Ultra-High Molecular Weight Polyethylene (UHMWPE) wear debris is thought to be a main factor in the development of osteolysis (1). However, the method for the evaluation of the biological response to UHMWPE particles has not yet been standardized.

In this study, four different types of UHMWPE particles were generated using a mechanized pulverizing method and the biological responses of macrophages to the particles were investigated using an inverted cell culturing process (2).

In search for appropriate materials of potential use to relieve injured articular cartilage, we explored copolymers from HEMA (2-hydroxy-methyl-methacrylate) and MMA (methyl-methacrylate). Such copolymers can be synthesized by thermal or photochemical induced polymerization reaction. The water uptake by swelling to homogeneous hydrogels can easily be controlled by varying the mixing ratio of the hydrophilic (HEMA) and hydrophobic (MMA) monomer, and the nature and amount of added crosslinker (typically EGDA, ethyleneglycol-dimethacrylate). Essentially the same variables strongly influence the mechanical properties, i.e. modulus (stiffness), relaxation response, as well as tribological behavior.

The polymer samples were engineered in molds from degassed formulations containing various amounts of HEMA and MMA, 10 % deionized water, and 0.01 % AIBN for thermal polymerization (12 h @ 70°C) or 0.5 % Darocur 1173 (2-hydroxy-2-methyl-1-phenyl-propane-1-one, for photopolymerization, 360 nm UV radiation, 5 to 7 min, sample thickness up to 5 mm). The samples were immersed in saline buffer after curing to allow free swelling to the equilibrium water content (EWC). Subsequently, samples were mechanically and tribologically tested. The mechanical moduli were determined at different strains and as a function of MMA content using a Zwicki Z5.0 (Zwick-Roell, Ulm, Germany). Tribological versus cartilage tissue was performed on an in-house-built pin-on-plate setup. Flat polymer samples were mounted and tested versus fresh porcine osteochondral grafts, harvested from humeral heads.

Mechanical testing revealed that the elastic modulus of pHEMA can be tuned as a function of MMA (0–50%) with 1 to 2 % bifunctional crosslinker to values ranging between 0.5 to 50 MPa, and corresponding water content of 40 to 10 % (decreasing with increasing MMA content). Friction measurements revealed a very low friction coefficient of around 0.02 for pHEMA-cartilage pairings. The values are 2–5 fold smaller than typical values of CoCrMo or UHMWPE versus cartilage.

Hydrogels from HEMA and MMA as main constituents are already rather well known for their biocompatibility. Knowledge of the dependence of e.g. the mechanical properties from chemical composition and polymer network structure makes this system ideal to design anisotropic specimen with controlled macrostructure to be used for temporal or permanent implants.

Introduction

Corrosion of the femoral head-trunnion junction in modular hip components has become a concern as the corrosion products may lead to adverse local tissue reactions. A simple way to avoid trunnion corrosion is to manufacture the femoral head with a non-metallic material, such as ceramics that are widely. An alternative solution may lie in advanced polymers like polyaryletherketones (PAEKs). These thermoplastics have high mechanical strength necessary for use as femoral heads in hip arthroplasty, but they must be tested to ensure that they do not adversely affect the wear of the ultrahigh molecular weight polyethylene (UHMWPE) liner counterface. Pin-on-disc (POD) wear testing has been extensively used to evaluate the wear properties of UHMWPE prior to more extensive and costly analysis with joint simulators. We hypothesized that the wear of crosslinked UHMWPE would not be adversely affected in POD tests when articulated against an advanced thermoplastic counterface.

Introduction

Vitamin-E (VE)-blended UHMWPE has been developed as a bearing-surface material due to the antioxidant ability of VE and has demonstrated a low wear rate in knee simulator [1]. Additionally, in vitro biological response testing has revealed that wear particles from VE blended UHMWPE induce the secretion of inflammatory cytokines at significantly lower levels compared to conventional UHMWPE [2]. However, as the joint kinematics are different between the knee and the hip, it is not guaranteed that these improvements will be repeated in the hip. In this study, the wear resistance of VE-blended UHMWPE was evaluated in knee and hip simulator tests and the effects of VE concentration and electron-beam irradiation were investigated.

Purpose

The objective of this study was to compare the wear characteristics and damage scores in highly crosslinked (XLPE) and conventional polyethylene (CPE) acetabular liners.